The subject matter disclosed within generally relates to coils for use in Magnetic Resonance Imaging (MRI) systems. MRI systems rely on both magnetic field and RF energy to create images. Generally, as the magnetic field strength increases, the optimum RF frequency increases proportionally. For example, the optimum RF frequency for a magnetic field strength of one Tesla (T) can be about 43 MHz. However, an optimum RF frequency for a magnetic strength of 3T can be about 128 MHz, and an optimum RF frequency for a magnetic strength of 7T can be about 300 MHz.
MRI systems generally require coils that can act as antennas to transmit and receive RF pulses. These coils can be referred to as transceivers due to their ability to both transmit and receive. However, the coils can also operate as receive-only or transmit-only antennas. The RF pulses are used to image a subject in an MRI. In order to further increase the image resolution of surface coil MRI, higher magnetic field strength MRI magnets can be used (3T and 7T). However, with this increased magnetic field strength, high RF frequencies are required. This higher RF frequency requirement can result in challenges related to operating in the ultrahigh field range (UHF, B0≥7T). UHF MRI of the human body is challenging due to the reduced wavelengths in tissue resulting in electro-magnetic (EM) field interferences, causing B1+ field inhomogeneities, reduced RF penetration and reduced transmit efficiency. Multichannel local transmit coil arrays have been developed to overcome some of the above challenges. For example, loop-coils, microstrip line elements, and dipole antennas.
Use of dipole antennae have become increasingly popular in UHF MRI and in deep tissue targets, as linear (i.e. electric dipole-like) current patterns are recognized as being favorable when considering ultimate signal-to-noise ration (SNR). Several coil arrays have been successfully used in body imaging applications by using different implementations of dipole antenna designs. Current designs rely on element separation to maintain adequate decoupling performance; a requirement that limits the number of elements can improve transmit performance by providing more degrees of freedom for shimming algorithms and can also provide a means to deliver higher peak power in systems using available hardware. This is important in UHF applications where demanding high peak B1+ acquisitions are needed and/or where deeply situated or large tissue targets are pursued, utilizing the full capabilities of the MR transmit hardware has utmost importance.
In a dipole antenna, the current distribution is symmetric along the long axis, whereas a loop-coil demonstrates an anti-symmetric current distribution. Due to these distinct current distribution patterns, a dipole antenna and a loop element can be decoupled from each other by carefully aligning the two elements along their center longitudinal axes. Combining electric dipoles with loop-coils has been proposed by Eryaman et al in simulation studies to reduce local specific absorption rates (SAR) in 7T head and spine imaging. Head coil arrays consisting of 8 loops and 8 dipoles on receive have been shown to improve the SNR at the central locations of the head compared to loop-only array designs. Previously, a 7T body imaging array with 8 fractioned dipole antenna transceivers along with 16 loop receivers and demonstrated improved SNR inside the prostate when both dipole and loop element were used on reception. However, dipole and loop-coil elements have not been combined to operate as transceivers (i.e. simultaneously transmit and receive).
Thus, it would be advantageous to have a method and apparatus that allows for hybrid dipole, loop-coil transceivers to improve SNR, B1+ transmit and SAR efficiencies in order to alleviate the B1 related challenges, particularly, those encountered at higher field strengths, such as 3T, 7T and above. However, the hybrid dipole, loop-coil transceivers discussed herein can also be used to improve SNR, B1+ transmit and SAR efficiencies in MRI systems operating at any field strength, including those less than 3T.